Black 19.10b0 no longer fixes short argument lists to a single line

Describe the bug

In previous versions of Black, short argument lists would be wrapped to a single line. In new versions of Black, this is not always the case.

If the argument list starts off as a single line, it’ll remain a single line. If the argument list starts as multiple lines, it’ll remain multiple lines. See test case below.

To Reproduce

test.py

my_func(
    "a",
    "b",
)

With Black 19.3b0

@@ -1,5 +1,2 @@
-my_func(
-    "a",
-    "b",
-)
+my_func("a", "b")

With Black 19.10b0

@@ -1,5 +1,4 @@
 my_func(
-    "a",
-    "b",
+    "a", "b",
 )

Expected behavior

I expect the behavior experienced in 19.3b0.

Environment

• Version: 19.10b0 (also master)
• OS and Python version: Fedora 30 Python 3.7.5

Does this bug also happen on master?

Yes.

https://black.now.sh/?version=master&state=_Td6WFoAAATm1rRGAgAhARYAAAB0L-Wj4ABjAEZdAD2IimZxl1N_WmBpTZzyXdYgGAumRPJ9fzc8wrFrsw5q41oEoZU7ytbygjdQqX-b0UdsSVEi1fbjjwWIr_X0wjnhZyweKwAAAABPNX1pNP1yrQABYmRPGZI9H7bzfQEAAAAABFla

[jdufresne]

FWIW, as an end user, I find this new beahvior quite suprising and a move away from the opinionated philosophy I really love about Black. When I write Python code, I almost never think about formatting anymore because I know Black will take of it later. With this change, it means I’m back to thinking about formatting. “Should I add a trailing comma or remove it?”, is now a question I’ll be asking before submitting code.

I normally blindly add a trailing comma to all code. Not because I want to keep arguments across multiple lines, but just out of habit. I don’t think about it much more than that and let Black figure it out later.

The PR that introduce this change is all about preserving multiple lines in collections. So what does that have to do with argument lists of function calls? Are you intentionally grouping argument lists in with collections here, or is that an unintended side effect of the implementation?

I think there is another problem with this change. Consider a very long argument list:

my_func("abcabcabc123123", "abcabcabc123123", "abcabcabc123123", "abcabcabc123123", "abcabcabc123123")

After formatting using master, this becomes (notice a trailing comma was added):

my_func(
    "abcabcabc123123",
    "abcabcabc123123",
    "abcabcabc123123",
    "abcabcabc123123",
    "abcabcabc123123",
)

Now – perhaps due to refactoring – the last 2 arguments are removed and reformatted:

my_func(
    "abcabcabc123123", "abcabcabc123123", "abcabcabc123123",
)

Now, because of this new change and the added trailing comma, Black doesn’t reformat the line back to a single line.

[JelleZijlstra]

This is intended behavior of the magic trailing comma. As of recent releases, you can use --skip-magic-trailing-comma if you don’t want this behavior.